And to make it even more confusing, there are also "Not Via London" fares set by VTEC, which are presumably intended to allow travel via Stevenage, but according to the Routeing Guide are only valid via Ely as well!

So far as I can see, the routeing guide allows Waterbeach to Peterborough either via Ely or via Hitchin. Both Cambridge and Ely are valid routeing points for Waterbeach for a Waterbeach-Peterborough journey (Peterborough is a routeing point), and Cambridge to Peterborough is permitted via Hitchin as well as via Ely.

So Cambridge only passes the fares rule if you have the Via Ely ticket, which means you can't go via Stevenage, NRE also says you need two tickets for that route.

Just to say, a Waterbeach-Peterborough "any permitted" ticket passes the fare check for Cambridge (or Ely) because ATOC's routeing point calculator says it does.

The maps mean that it's valid via Cambridge and Hitchin.

Easement 700070 says "Customers travelling from stations North of Cheshunt via Cambridge and Hitchin to stations North of Hitchin may double back between Hitchin and Stevenage. This easement applies in both directions." This means that it's valid via Cambridge and Stevenage.

Just to say, a Waterbeach-Peterborough "any permitted" ticket passes the fare check for Cambridge (or Ely) because ATOC's routeing point calculator says it does.

The maps mean that it's valid via Cambridge and Hitchin.

Easement 700070 says "Customers travelling from stations North of Cheshunt via Cambridge and Hitchin to stations North of Hitchin may double back between Hitchin and Stevenage. This easement applies in both directions." This means that it's valid via Cambridge and Stevenage.

I don't think a "via Ely" ticket gives any extra validity.

Presumably, someone could go waterbeach to Cambridge, and then Cambridge to Peterborough on the XC

They can, and they can also be used with "any permitted" tickets. Cambridge-Ely-Peterborough is a valid route from Cambridge to Peterborough. It isn't "a route not listed in the Routeing Guide for which a higher priced route specific fare exists".